Download Shrimps – a nutritional perspective

GENERAL ARTICLES
Shrimps – a nutritional perspective
J. Syama Dayal*, A. G. Ponniah, H. Imran Khan, E. P. Madhu Babu, K. Ambasankar and
K. P. Kumarguru Vasagam
This article presents the nutritional value of shrimp on the strength of its nutrient composition
and daily value (DV%). With its relatively lower lipid content (~ 1%), the DV (%) of 100 g shrimp
for an adult human is 75%, 70% and 35% for eicosapentanoic acid + docosahexanoic acid, essential amino acids (methionine, tryptophan and lysine) and protein respectively. The lower atherogenic (0.36) and thrombogenic (0.29) indices of shrimp show its cardio-protective nature. The
controversy relating to shrimp cholesterol and the overall health benefits of eating shrimp are discussed to indicate that shrimp should be a regular item in the diet of normolipidemic peoples.
Keywords:
Daily value, dietary cholesterol, nutritional composition, shrimp.
FOOD helps human beings maintain good health by providing all essential nutrients. Consuming a variety of
foods in balanced proportions will prevent deficiency
diseases and chronic diet-related disorders. Shrimp is one
of the most delicious seafoods and is part of almost every
nation’s traditional meal. Its popularity has created a
demand for its produce around the world. Considering the
limit in the supply from wild catch, farming of shrimp
was started in many countries to meet the demand, and is
growing rapidly. One can gauge its growth from the past
production figures which rose from 1 million tonnes (mt)
in 2001 to 4.02 mt in 2010, including the Indian contribution of about 0.1 mt (ref. 1). The potential for growth of
Indian shrimp is exhibited by increase in its exports by
24% in quantity and 42% by value in 2012 compared to
2011 (ref. 2). Shrimp production is expected to grow
more than 50% globally during 2010–2030, indicating its
vibrant production potential3.
Fresh and clean shrimps can be served either cooked or
uncooked with sauce. From a nutritional standpoint,
shrimps are high in protein, low in saturated fat4 and
calories, and have a neutral flavour. Due to these characteristics, shrimps form a natural additive in salads, pastas,
curry, soups and stir-fried dishes. Shrimps have also been
identified as a rich source of vitamin B12, selenium, ω-3
highly unsaturated fatty acids (HUFA) and astaxanthin, a
potent natural antioxidant5. Despite the several nutritional
parameters of shrimp based on which it can be considered
as a healthy food, there is reluctance among dieticians
and health professionals as well as consumers because
of its relatively higher cholesterol6–8. A clinical study
showed that moderate shrimp consumption in normolipidemic subjects will not adversely affect the overall lipoThe authors are in the Central Institute of Brackishwater Aquaculture,
No. 75, Santhome High Road, R.A. Puram, Chennai 600 028, India.
*For correspondence. (e-mail: [email protected])
CURRENT SCIENCE, VOL. 104, NO. 11, 10 JUNE 2013
protein profile and can be included in ‘heart healthy’
nutritional guidelines9. Lack of scientific studies directly
linking shrimp consumption to cardiovascular disease
(CVD) has left the dieticians and doctors in a predicament. Egg from poultry has been identified as a highcholesterol food item for human and its consumption has
been a matter of debate for cardiovascular health. Unlike
shrimps, many studies have been carried out on the effect
of egg consumption on serum lipid profile in relevance to
CVD and other metabolic disorders in humans. However,
the argument on cholesterol-rich egg consumption and
its relation on CVD risk has not yet been conclusively
settled. A review by a medical doctor and co-workers
criticized the indiscriminate consumption of eggs by
adults10. Another recent review argues that dietary cholesterol does not increase the risk of heart disease in
healthy populations, and therefore suggests the withdrawal of recommended dietary intake (RDI) of less than
300 mg/day (in USA)11. Several studies agree that cholesterol in egg does not contribute much to blood cholesterol, and support the idea of eating an egg each day.
An attempt has been made in this article to evaluate the
nutritional value of shrimp in man’s platter, mainly
focusing on its nutritional composition, daily value of
major nutrients and dietary cholesterol. This may put to
rest the negative perceptions and controversies on shrimp
cholesterol and might pave the way for accepting it as a
nutritional food for healthy human beings6,7,9,12. The
shrimps (Penaeus monodon and Fenneropenaeus indicus)
used for analysis in our laboratory were in the size range
10–25 g collected from both wild caught and cultured.
Protein and other vital nutrients of human
importance in shrimp
Similar to any animal meat, shrimp is an excellent source
of dietary protein. The analysed nutrients in the eatable
1487
GENERAL ARTICLES
muscle of shrimp are presented in Table 1. Three-fourths
of the edible portion of shrimp are water. Nearly 80% of
the remaining portion (dry matter) comprises of protein.
The average protein content of fresh shrimp is 19.4 g/
100 g and it contributes 87% of the total energy. Our
body cannot synthesize certain amino acids and they must
be obtained through diet; these are called essential amino
acids. The protein digestibility corrected amino acid score
(PDCAAS) is based on the amino acid content of food
protein, true digestibility, and its ability to supply the essential amino acids according to requirement. The
PDCAAS of shrimp is 1, indicating its superior protein
Table 1. Average analysed nutrient profile of 100 g edible shrimp meat*
Nutrient
Protein (g)
Lipid (g)
Water (g)
Energy (kcal)
Essential amino acids (mg)
Isoleucine
Leucine
Lysine
Methionine + cysteine
Phenylalanine + tyrosine
Threonine
Tryptophan
Valine
19.4 ± 0.56
1.15 ± 0.19
76.3 ± 0.57
89.0 ± 1.12
930.7 ± 8.10
1463.9 ± 22.30
1480.1 ± 27.57
668.1 ± 16.57
1389.2 ± 19.27
756.0 ± 8.89
223.3 ± 2.90
935.7 ± 5.89
Lipid composition
∑SFA (mg)
∑MUFA (mg)
∑PUFA (mg)
Eicosapentaenoic (mg)
Docosahexaenoic (mg)
∑n – 3 PUFA (mg)
∑n – 6 PUFA (mg)
n – 6/n – 3 PUFA
PUFA/SFA
Cholesterol (mg)
257.5 ± 3.71
163.5 ± 7.90
321.0 ± 5.23
112.0 ± 3.02
75.5 ± 1.43
204.5 ± 2.23
106.0 ± 2.31
0.5 ± 0.01
1.3 ± 0.05
173 ± 6.93
Macro minerals (mg)
Calcium
Magnesium
Phosphorus
Potassium
Sodium
107.3 ± 1.96
58.5 ± 1.38
303.4 ± 3.22
259.6 ± 3.25
176.1 ± 3.04
Micro minerals (μg)
Copper
Iron
Manganese
Selenium
Zinc
918 ± 4.62
2196.5 ± 16.61
50.5 ± 1.64
44 ± 1.06
1403.5 ± 5.43
*These are the average values (n = 60) with SEM of nutrients from the
data obtained for tiger shrimp (Penaeus monodon) and Indian white
shrimp (Fenneropenaeus indicus) from both wild caught (10–20 g) and
cultured (20–25 g). The average proximate composition (%) of feeds
used is 9.55, 38.32, 6.30, 3.93, 12.51 and 29.39 for moisture, crude protein, ether extract, crude fibre, total ash and nitrogen-free extract
respectively.
1488
quality. The fullness factor is 3.3 on a 0–5 scale, indicating that it provides more essential nutrients per calorie
and can be considered a healthy food choice like fish13.
Another advantage of eating shrimp is its significant
lower lipid content. Analysed lipid levels in shrimp were
around 1.15 g/100 g. No other meaty food can claim such
a low lipid level as fresh shrimp. The lipid class composition of shrimp comprises of 65–70% phospholipids, 15–
20% cholesterol and 10–20% total acyl glycerols. The predominance of phospholipids in shrimp lipid indicates its
rich nutritional quality, which is an integral part of cell
membrane and transport lipoproteins. Furthermore, 32%
of shrimp lipid is composed of poly unsaturated fatty acids (PUFA), a term usually associated with high-quality
seafood. Within this, about 64% is contributed by ω-3
PUFA and the remaining 33% by ω-6 PUFA, making a
ω-6/ω-3 PUFA ratio around 1.9, indicating high health
benefits. The ω-6/ω-3 PUFA ratio is a critical
dietary factor due to its decisive role in balancing the
eicosanoid synthesis14. A healthy diet should have a
PUFA/SFA ratio of at least 0.54 and above15. The
shrimps assessed in our study had higher values (> 1.9)
than the recommend ratio and are definitely a valuable
source of PUFA. Shrimp is also beneficial due to its mild
anti-inflammatory nature because of its high selenium,
docosahexanoic acid and low saturated fatty acids13.
Epidemiological studies revealed that consumption of
seafood, including shrimp rich in ω-3 PUFA is associated
with a reduced risk of coronary heart diseases16,17 and
cancer18. The US Dietary Guidelines recommend that
individuals at both higher and average CVD risk consume
an average of at least 250 mg/day EPA + DHA (1750 mg/
week)19. Eating 100 g edible portion of shrimp in a day
will provide > 180 mg of EPA + DHA. In order to measure the propensity of shrimp-eating in reducing the risks
of coronary heart disease, the atherogenic and thrombogenic indices were calculated20. Shrimp exhibited values of 0.36 and 0.29 for atherogenic and thrombogenic
indices respectively, which are lower than other nonvegetarian foods, indicating its cardio-protective nature.
Shrimps were also found to be a rich source of astaxanthin, a lipid soluble carotenoid formed from ingested
β-carotene or zeaxanthin through oxidative transformation5,21. Astaxanthin has been found to be a potent natural
antioxidant, exceeding ten times the antioxidant activity
of β-carotene and 500 times that of α-tocopherol5. The
astaxanthin level of wild shrimps has been reported to
vary between 740 and 1400 μg/100 g in edible meat portions21, which again supports the argument for including
them as part of the daily diet.
Human diet requires minerals such as calcium, phosphorus, magnesium, potassium and sodium in large amounts
and hence these are referred to as macro minerals. A 100 g
serving of shrimp provides > 100 mg of calcium, > 300 mg
of phosphorus and > 40 μg of selenium. Among their numerous functions, minerals help regulate the fluid balance,
CURRENT SCIENCE, VOL. 104, NO. 11, 10 JUNE 2013
GENERAL ARTICLES
Figure 1.
Daily value (DV%)* of one serving (100 g) of shrimp for important nutrients.
enzyme production and bone health. Eating shrimp
(100 g/day) would provide approximately ten vitamins
and ten minerals. Shrimp contains important vitamins like
vitamin A (180 IU), vitamin D (2 IU) and vitamin E
(1.32 μg), vitamin B12 (1.11 μg) and vitamin B3
(1.77 mg)22. All these nutrients are known to exhibit considerable level of variation in relation to molting stage of
shrimp23. Hence the shrimps used for analysis in our
laboratory were in post-molt stage. Within the marketable
size range, much variation is not observed in macro and
micro nutrients. The cultured tiger shrimps were sampled
from commercial ponds with standard management
culture practices without any disease outbreaks and natural calamities.
Shrimp consumption in relation to RDA of
nutrients
The recommended daily allowance (RDA) projects the
quantitative recommendation of nutrients for people in
general to stay healthy. This may be different for different categories like children, adult males and females. The
Indian Council for Medical Research (ICMR) favours the
FAO/WHO/UNU guidelines for framing the RDA guidelines, with slight modifications. The RDA for a nutrient
will be in a standard unit which helps the common man to
CURRENT SCIENCE, VOL. 104, NO. 11, 10 JUNE 2013
easily calculate his own requirements based on the body
weight and/or basal metabolic rate. The RDA can be used
to calculate the daily value of foods (DV%). Hundred
grams of shrimp has been taken to calculate the DV%
that was recommended as serving size of meat24. For
example, the RDA of protein is measured as intake/kg body
weight, as 0.8 g/kg of individual body weight25. It means
that an individual weighing 70 kg would need to take
56 g (0.8 × 70) of protein daily. Considering this RDA,
consumption of 100 g shrimp containing 19.4 g protein
by an individual weighing 70 kg would fulfil 35% of the
daily protein requirement. This is the DV% of 100 g
shrimp with respect to protein. Similarly, one can compute the DV% for the nutrients considering the requirement and content of a particular nutrient in the selected
food item. Figure 1 shows the calculated DV% for important nutrients in 100 g shrimp for a 70 kg adult man.
Using this chart one can obtain a quick assessment for
selecting shrimp for his/her meal in a day. When shrimps
are scored in the DV% chart of nutrients, the following is
observed: DV% > 70 is rated outstanding – selenium,
methionine, tryptophan, threonine, EPA + DHA and lysine;
DV% 50–70 (excellent) – copper, isoleucine, cystine, histidine, leucine, valine and phosphorus; DV% 25–50 (very
good) – protein and phenylalanine; DV% 10–25 (good) –
calcium, magnesium, iron and zinc; DV% < 10 (poor) –
sodium, potassium, energy, lipid and manganese.
1489
GENERAL ARTICLES
Table 2.
Comparison of shrimp, egg and meat* on their cholesterol, saturated fatty acids and
atherogenic index
Non-vegetarian food
Shrimp
Egg
Chicken
Mutton
Beef
Pork
Health significance
SFA@ (g/100 g)
0.25
4.0
6.0
7.0
8.0
13.0
Lowest in shrimp;
good for health
Cholesterol# (mg/100 g)
173
400
100
65
70
90
Moderate in shrimp;
but not harmful
due to low SFA
Atherogenic index$
0.36
0.40
0.50
1.00
0.70
0.67
Lowest in shrimp;
good for health
*Dietary guidelines24,25,30. @Higher saturated fatty acids consumption increases blood cholesterol.
#
Cholesterol consumption should not be more than 300 mg (USA) per day. $Lower atherogenic index is good
for a healthy heart.
Dietary cholesterol controversies and shrimp
consumption
Cholesterol is an important sterol that plays a prominent
role in membrane structure and is also a precursor for the
synthesis of steroid hormones and bile acids in man. A
relatively constant level of cholesterol in the blood (150–
200 mg/dl) is maintained primarily by controlling the
level of de novo synthesis. The synthesis and utilization
of cholesterol must be tightly regulated in order to prevent CVD. Reduction in circulating cholesterol levels can
have a profound positive impact on CVD. It is long been
presumed that control of dietary intake is one of the easiest means to reduce cholesterol. The cholesterol, SFA
and atherogenic indices of shrimp, egg and other nonvegetarian meats are given in Table 2 in relation to the
ICMR dietary guidelines25. Though shrimps are adequate
in many essential nutrients, their higher cholesterol level
and with the consumers’ perception of the link between
dietary cholesterol and CVD, medical practitioners and
dieticians have placed shrimp in avoidable food item
ignoring its overall lipid and fatty acid profiles6,7. Moreover, there is no sound scientific evidence to support this
assumption, in particular, the relation between shrimp
cholesterol level and occurrence of CVD. A clinical trial
comparing the serum lipid profile of volunteers who ate
shrimp and eggs, concluded that shrimp consumption in
normolipidemic people would not adversely affect the
overall lipoprotein profile and shrimp can be included in
‘heart healthy’ nutritional guidelines9. A recent review
based on epidemiological and clinical studies from the
past 20 years, claims that dietary cholesterol does not
increase the risk for heart disease in healthy populations11. Studies in support of a direct relationship between
dietary cholesterol and increased risk for heart disease are
limited, and they mostly demonstrate that diabetic individuals are at risk. Citing this as evidence, withdrawal of RDI
of less than 300 mg/day (in USA) of dietary cholesterol
was suggested, since this is restricting the consumption of
eggs, otherwise a good source of quality protein, carote1490
noids and choline. Further evidence was provided by a
challenge trial in which adults were allowed to consume
640 mg of daily cholesterol for four weeks. This study
identified that in 75% of individuals there was limited or
no increase in plasma cholesterol level. However, in 25%
of the individuals, there was blood cholesterol elevation
consequent to high cholesterol intake. Surprisingly, the
increase was observed for both LDL and HDL cholesterol
with no changes in the LDL/HDL cholesterol ratio, a key
marker of CVD risk11. Consumption of a high-cholesterol
diet with high saturated fat results in the increase of LDL
level rather than diets with high PUFA26. This indicates
the necessity to look at the overall dietary lipid influence
on serum lipids rather than cholesterol alone. In another
clinical study with type-2 diabetic patients, low-cholesterol
beef was replaced with cholesterol-rich poultry meat;
18% reduction in serum total cholesterol was observed.
This study also concluded that significant reduction in
cholesterol may be due to the higher PUFA and lower
SFA proportions in poultry meat compared to beef27. This
supports the notion that the type of dietary fatty acid,
rather than the level of dietary cholesterol, is the most potent regulator of serum cholesterol levels28. Another reason to encourage shrimp consumption is the potential
association between hypotriglyceridemic ω-3 PUFA and
CVD17, shrimp being an excellent source of EPA + DHA
(DV > 70%). It was concluded in a human clinical trial
that shrimp when consumed in moderation can play a role
in a heart-healthy diet, because it does not markedly
affect the ratio of ‘good’ HDL to ‘bad’ LDL cholesterol9.
Therefore, consuming 100 g of shrimp even on a daily
basis would only supply less than the RDA (< 300 mg/day)
of cholesterol. Furthermore, an experiment with hypercholesterolemic rat models documented the cholesterollowering effect of the extract of salted and fermented
shrimp12. Another study with a rat model reported the
serum lipid-lowering effects of Antarctic krill oil on
dietary intake29. Antarctic krill is a small swimming crustacean, which is considered to be the ancestor of shrimp
and prawn.
CURRENT SCIENCE, VOL. 104, NO. 11, 10 JUNE 2013
GENERAL ARTICLES
Shrimps have low fat, less cholesterol and high PUFA
content compared to eggs, and the current understanding
on dietary cholesterol linked with egg consumption
clearly confirms the nutritional value of shrimps. Based
on the above-mentioned evidences on health-related outcomes, the nutrient profiles analysed in this study and a
detailed appraisal of the RDA of national and international agencies, it can be concluded that shrimp can be
considered as healthy food for humans. The health benefits of eating shrimp go beyond ω-3 fatty acids and other
individual nutrients; it is the effect of the combined nutrients present in shrimp, including proteins, trace minerals
and carotenoids. Therefore, to benefit from vital nutrients
like ω-3 HUFA, astaxanthin, protein, essential amino
acids, vitamins and minerals, a moderate quantity of
shrimp should be consumed by people. Only those who
are at a clear risk of CVD and diabetics may avoid this,
until the dietary cholesterol controversy is settled.
1. Food and Agricultural Organization, The State of World Fisheries
and Aquaculture, Bi-Annual Report, FAO, Rome, 2012, p. 209.
2. The Marine Products Export Development Authority, Marine
products exports crossed 3.5 billion mark in 2011–12. Ministry of
Commerce, Government of India, Cochin, 2012; http://www.
mpeda.com/press1.pdf
3. Larkin, T., Aquaculture global outlook and sustainability. In 24th
Annual North Carolina Aquaculture Development Conference,
Raleigh, NC, USA, 2012.
4. Dayal, J. S., Ponniah, A. G. and Ambasankar, K., Food value of
shrimp. In 9th Indian Fisheries Forum – Renaissance in Fisheries:
Outlook and Strategies Souvenir (eds Ramachandran, C. et al.),
Organized by Asian Fisheries Society (Indian Branch) hosted by
Central Marine Fisheries Institute, Chennai, 19–23 December
2011, pp. 1–72.
5. Venugopal, V. (ed.), In Marine Products for Healthcare: Functional and Bioactive Nutraceutical Compounds from the Ocean,
CRC Press, London, 2009, pp. 221–239.
6. Robinson, C. H., The low cholesterol, low fat diet. Am. J. Clin.
Nutr., 1954, 2, 353–355.
7. Assmann, G. et al., Coronary heart disease: reducing the risk: a
worldwide view. Circulation, 1999, 100, 1930–1938; doi:
10.1161/01.CIR.100.18.1930.
8. Carney, C., Is shrimp high in cholesterol; http://www.ehow.com/
facts_5474631_shrimp-high-cholesterol.html
9. De Oliveira e Silva, E. R., Seidman, C. E., Tian, J. J., Hudgins,
L. C., Sacks, F. M. and Breslow, J. L., Effects of shrimp consumption on plasma lipoproteins. Am. J. Clin. Nutr., 1996, 64, 712–
717.
10. Spence, J. D., Jenkins, D. J. A. and Davignon, J., Dietary cholesterol and egg yolks: not for patients at risk of vascular disease.
Can. J. Cardiol., 2010, 26, 336–339.
11. Fernandez, M. L., Rethinking dietary cholesterol. Curr. Opin.
Clin. Nutr. Metab. Care, 2012, 15, 117–121.
12. Seok, S. H., Park, J. H., Choa, S. A., Choi, S. A. and Park, J. H.,
Cholesterol lowering effect of SG-GN3, the extract of salted and
fermented small shrimps, Acetes japonicus, in Triton WR-1339 or
high cholesterol-diet induced hypercholesterolemic rats. J. Ethnopharmacol., 2004, 91, 231–235.
CURRENT SCIENCE, VOL. 104, NO. 11, 10 JUNE 2013
13. Nutrition Data, Nutrition facts, crustaceans, shrimp, mixed species, cooked, moist heat. http://nutritiondata.self.com/facts/finfìshand-shellfish-products/4174/2
14. Simopoulos, A. P., The importance of the omega-6/omega-3 fatty
acids ratio in cardiovascular disease and other chronic diseases.
Exp. Biol. Med., 2008, 233, 674–688.
15. Health Education Authority, Nutritional Aspects of Cardiovascular
Disease, Hamilton House, London, 1996, pp. 13–17.
16. Kris-Etherton, P. M., Harris, W. S. and Appel, L. J., Fish consumption, fish oil, omega-3 fatty acids and cardiovascular disease.
Circulation, 2002, 106, 2747–2757.
17. Mozaffarian, D. and Wu, J. H., Omega-3 fatty acids and cardiovascular disease: effects on risk factors, molecular pathways and
clinical events. J. Am. Coll. Cardiol., 2011, 58, 2047–2067.
18. Murphy, R. A., Mourtzakis, M. and Mazurak, V. C., n – 3 Polyunsaturated fatty acids: the potential role for supplementation in
cancer. Curr. Opin. Clin. Nutr. Metab. Care, 2012, 15, 246–251.
19. US Department of Agriculture and US Department of Health and
Human Services. Dietary Guidelines for Americans, US Government Printing Office, Washington, DC, December 2010, 7th edn.
20. Ulbright, T. L. and Southgate, D. A. T., Coronary heart diseases:
seven dietary factors. Lancet, 1991, 338, 985–992.
21. Sachindra, N. M., Bhaskar, N. and Mahendrakar, N. S., Carotenoids in different body components of Indian shrimps. J. Sci.
Food Agric., 2005, 85, 167–172.
22. USDA, National Nutrient Database for Standard Reference
Release 24. Nutrient Data Laboratory, Beltsville, MD, 2012;
http://www.ars.usda.gov/main/site_main.htm?modecode=12-3545-00
23. Chang, E. S., Physiological and biochemical changes during the
molt cycle in decapod crustaceans: an overview. J. Exp. Mar. Biol.
Ecol., 1995, 193, 1–14.
24. Enser, M., Hallet, K., Hewitt, B., Fursey, G. A. J. and Wood,
J. D., Fatty acid content and composition of English beef, lamb
and pork at retail. Food Chem., 1996, 42, 443–456.
25. Indian Council of Medical Research, Nutrient requirements and
recommended dietary allowances for Indians. ICMR, New Delhi,
2009, p. 334.
26. Fielding, C. J., Havel, R. J. and Todd, K. M., Effects of dietary
cholesterol and fat saturation on plasma lipoproteins in an ethnically diverse population of healthy young men. J. Clin. Invest.,
1995, 95, 611–618.
27. Gross, J. L. et al., Effect of a chicken based diet on renal function
and lipid profile in patients with type 2 diabetes. Diabetes Care,
2002, 25, 645–651.
28. Schaefer, E. J., Lipoproteins, nutrition, and heart disease. Am.
J. Clin. Nutr., 2002, 75, 191–212.
29. Zhu, J. J., Shi, J. H., Qian, W. B., Cai, Z. Z. and Li, D., Effects of
Krill Oil on serum lipids of hyperlipidemic rats and human SW480
cells. Lipids Health Disease, 2008, 7, 30.
30. Ghafoorunissa and Kamala, K., Diet and Heart Disease. National
Institute of Nutrition, Hyderabad, 2007, p. 74.
ACKNOWLEDGEMENTS. We thank Dr S. Ayyappan, Director
General, Indian Council of Agricultural Research (ICAR), New Delhi
for his encouragement under ‘ICAR Outreach Activity on Nutrient Profiling and Evaluation of Fish as a Dietary Component’. The support
from Dr B. P. Mohanty, Coordinator and Dr T. V. Sankar, Cocoordinator of this network program is acknowledged. We also thank
Dr G. Gopikrishna for valuable suggestions that helped us improve the
manuscript.
Received 27 August 2012; revised accepted 1 April 2013
1491